Be honest. There is nothing worse then saying something is great, when it's not. Honest feedback will ultimately help you to give a good critique.
Be detailed in your assessment. When giving feedback, draw on your design vocabulary to help you articulate why you felt the work was not successful. The more detail you can give, the better. It helps your peer understand where his or her work went wrong.
Offer constructive criticism. Don't be mean when reviewing someone's work. People can very sensitive about their work, and when you are criticizing something they may have spent days working on, it is very easy for them to take it personally. As you critique, try to offer solutions. Suggest ways that they can correct the misstep. This is the most rewarding thing to do, as not only are you aiding a peer in their work, but you are pushing your own creativity to come up with creative solutions to a problem. Creative problem solving is major facet of design.
Be open minded. Just because your peer's style may be different from your own does not mean it is bad. You need be open to other solutions to a problem.
Ask questions. Don't be afraid to ask your peers questions about what the work means or what they were trying to say. Also, when accepting criticism, do not be content with simple one word comments. If someone offers his or her opinion and you do not understand it, ask for clarification -- get to the heart of what they were trying to say.